Intraspecific Facilitation Research Articles

Population re-establishment and spatial dynamics of crowberry (Empetrum nigrum ssp. hermaphroditum), a foundation species in restored alpine ecosystems.

Many ecosystems are defined and shaped by one or a few common, foundation species. Even though such species hold a key role in the restoration of these ecosystems, the demographic processes involved in their re-establishment have rarely been studied. Foundation species' population dynamics, re-establishment history, and the abiotic and biotic factors that affect individual establishment at restored sites can be studied by addressing population spatial patterns and age structure. Such an approach to studying population dynamics is particularly relevant for long-lived species with low mortality, such as shrubs in alpine areas. We studied a population of the foundation species Empetrum nigrum ssp. hermaphroditum at an alpine spoil heap site and found evidence of population re-establishment starting within a decade after construction. High Empetrum densities close to the spoil heap edges indicated that short distances to seed sources in the surroundings had a strong positive effect on establishment of individuals. Empetrum individuals were significantly clustered, which indicated intraspecific facilitation. As revealed by spatial analyses of recruits and older, established individuals, clustering developed gradually over time, which indicated a shift from no interaction to increased facilitation. We conclude that intraspecific facilitation promotes Empetrum reestablishment at the studied alpine spoil heap. Synthesis: We show that population spatial patterns and age structure can be successfully used to unveil the re-establishment history of a foundation species in a restoration context. Efficient seed dispersal and intraspecific facilitation seem to be important factors behind Empetrum's successful re-establishment at alpine spoil heaps. Identification of abiotic and biotic factors determining foundation species' establishment success at restored sites can support planning and improve success of restoration.

Facilitation drives tree seedling survival at alpine treelines

Abstract Alpine treelines are considered ecological monitors recording the impacts of climate change on trees and forests. To date, most treeline research has focused on how climate change drives treeline dynamics. However, little is known about how biotic interactions mediate treeline shifts, particularly in the case of tree recruitment, a bottleneck of treeline dynamics. We hypothesized that inter- and intraspecific facilitation determined the establishment and survival of tree seedlings at alpine treelines. To test this hypothesis, 630 Abies georgei var. smithii seedlings with different ages (4–6, 7–9 and 10–15 years old) were transplanted into three growth habitats (canopy-in, canopy-out and meadow) across the alpine treeline ecotone (4300–4500 m) in the Sygera Mountains, on the southeastern Qinghai-Tibetan Plateau. Microclimate, height growth, mortality rates and leaf functional traits of transplanted seedlings were measured over 3 years. We found that the variations in leaf functional traits were driven by microclimate. After the transplantation, the leaf concentrations of soluble sugars and starch and C:P ratio increased, whereas leaf size decreased. The resource use of seedlings gradually shifted to a more conservative strategy as indicated by changes in non-structural carbohydrates and nutrient concentrations. Radiation, temperature and moisture conditions, mediated by plant interactions, influenced seedling mortality and annual growth by affecting leaf morphological traits. Our findings illustrate how facilitation plays a crucial role in altering solar radiation and leaf trait functioning, determining seedling survival and growth at alpine treelines. We provide new insights into the underlying mechanisms for tree establishment and alpine treeline shifts in response to climate change.

Intraspecific Facilitation Through Host Plant Quality Improvement? Observations on the Aggregation Behaviour and Population Biology of the Senescence Inducing Phloem Feeder (Ischnodemus Sabuleti)

The aggregation behaviour and the senescence inducing effect on its host plants (Glyceria spp.) of the European chinch bug (Ischnodemus sabuleti) is documented in several Hungarian high- density populations. The hypothesis of intraspecific facilitation through host plant quality improvement is formulated and discussed in detail. We suggest that drought stressed host plants improve the performance of I. sabuleti and facilitate the local population outbreaks. We suggest that the optimal host plant drought stress level is higher for senescence feeding herbivores than for the flush feeding herbivores.

Feeding behavior and species interactions increase the bioavailability of microplastics to benthic food webs

Plastics are pervasive in aquatic ecosystems, in which they circulate in the water column, accumulate in sediments, and are taken up, retained, and exchanged with their biotic environment via trophic and non-trophic activities. Identifying and comparing organismal interactions are a necessary step to improve monitoring and risk assessments of microplastics. We use a community module to test how abiotic and biotic interactions determine the fate of microplastics in a benthic food web. Using single-exposure trials on a trio of interacting freshwater animals (the quagga mussel Dreissena bugensis, a filter feeder; the gammarid amphipod Gammarus fasciatus, a deposit feeder; and the round goby Neogobius melanostomus, a benthivorous fish), we quantify the (1) uptake of microplastics from environmental routes (water, sediment) under six exposure concentrations, (2) the depuration capacities over 72 h, and (3) the transfer of microbeads via trophic (predator-prey) and behavioral interactions (commensalism, intraspecific facilitation). Under 24 h exposures, each animal of our module acquired beads from both environmental routes. The body burden of filter-feeders was higher when they were exposed to particles in suspension, whereas detritivores had similar uptake from either route. Mussels transferred microbeads to amphipods, and both invertebrates transferred beads to their mutual predator, the round goby. Round gobies generally displayed low contamination from all routes (suspension, sedimented, trophic transfer) with a higher microbead load from preying on contaminated mussels. Higher mussel abundance (10–15 mussel per aquaria, i.e., ~200–300 mussels·m2) did not increase individual mussel burdens during exposure, and neither did it increase the transfer of beads from mussels to gammarids via biodeposition. Our community module approach revealed that the feeding behavior of animals allows microplastic uptake from multiple environmental routes, whereas trophic and non-trophic species interactions increased their burden within their food web community.

Species traits interact with stress level to determine intraspecific facilitation and competition

AbstractQuestionsFlooding and drought stress are expected to increase significantly across the world and plant responses to these abiotic changes may be mediated by plant–plant interactions. Stress tolerance and recovery often require a biomass investment that may have consequences for these plant–plant interactions. Therefore, we questioned whether phenotypic plasticity in response to flooding and drought affected the balance between competition and facilitation for species with specific adaptations to drought or flooding.LocationUtrecht University.MethodsStem elongation, root porosity, root:shoot ratio and biomass production were measured for six species during drought, well‐drained and submerged conditions when grown alone or together with conspecifics. We quantified competition and facilitation as the ‘neighbour intensity effect’ directly after the 10‐day treatment and again after a seven‐day recovery period in well‐drained conditions.ResultsWater stress, planting density and species identity interactively affected standardized stem elongation in a way that could lead to facilitation during submergence for species that preferably grow in wet soils. Root porosity was affected by the interaction between neighbour presence and time‐step. Plant traits were only slightly affected during drought. The calculated neighbour interaction effect indicated facilitation for wetland species during submerged conditions and, after a period to recover from flooding, for species that prefer dry habitats.ConclusionsOur results imply that changing plant–plant interactions in response to submergence and to a lesser extent to drought should be considered when predicting vegetation dynamics due to changing hydroclimatic regimes. Moreover, facilitation during a recovery period may enable species maladapted to flooding to persist.

Competition rather than facilitation affects plant performance across an abiotic stress gradient in a restored California salt marsh

The Stress Gradient Hypothesis predicts facilitation will become more important than competition where abiotic stress is high, and the framework successfully predicts positive interactions between species in many systems. Fewer studies have focused on intraspecific facilitation, and to our knowledge none examine intraspecific interactions in Pacific Coast salt marshes of North America. We used two species that tend to occur in large, monospecific patches to test for intraspecific facilitation in a restored California marsh, where tides and evaporation during summer create moisture and salinity gradients across elevation. We tested performance of Frankenia salina or Jaumea carnosa in restoration plots using two treatments: clustered plantings to promote facilitation, and widely spaced plantings to limit interaction between individuals. We characterized the soil water potential gradient and measured plant survival, cover, physiology, and susceptibility to herbivory across elevation and planting treatments. Soil water potential declined sharply with elevation, suggesting plant stress should be higher upslope. However, tissue water potential was unaffected by elevation, and survival was high—suggesting growing conditions remained benign. A seawater addition treatment did not alter the response of plants or plant interactions to the abiotic gradient. At 19 months, intraspecific competition prevailed across the entire elevation gradient for both species, with less transplant cover in clustered plantings. However, clustered Frankenia fared better during a transient period of heavy rabbit herbivory. Aside from this temporary benefit, clustering did not reduce stress and strongly suppressed growth—suggesting that restoration designs for the high marsh should minimize competition.

Opposite Tree-Tree Interactions Jointly Drive the Natural Fir Treeline Population on the Southeastern Tibetan Plateau

The long-term stability of alpine treeline positions and increased stem density are frequently reported by recent studies; however, whether a denser treeline forest is relevant to competitive tree–tree interactions remain unclear. Herein, we mapped and surveyed individual trees in two undisturbed Smith fir (Abies georgei var. smithii) treeline plots (with a size: 30 m × 200 m; plot NE1: 4477 m, NE2: 4451 m) near Ranwu Lake (RW) on the southeastern Tibetan Plateau. The surface pattern method and spatial point pattern analysis were used to detect the spatial distribution patterns of three size classes (seedlings, juveniles, adults) and spatial associations between the pairwise size classes. We also compared our results to the spatial patterns of the five other treeline forests (Deqin, Linzhi, Changdu, Yushu, Aba) reported from the Tibetan Plateau. Young trees dominated the two fir treeline plots. Both positive and negative spatial autocorrelations for all of the trees were detected in two study plots. Intraspecific facilitation and competition coexisted at the fir treelines in three forest regions (RW, Linzhi, Aba) characterized by a mild moist climate, whereas intraspecific facilitation dominated the other three forest regions (Changdu, Deqin, Yushu), which featured seasonal climatic stress or high disturbance pressure. Thus, increased stem density at alpine treeline can be linked to competitive interactions in relatively favorable environmental conditions. Overall, the spatial patterns of the treeline population are mainly shaped by the combination of thermal and moisture conditions and are also modulated by non-climatic variables (e.g., disturbance history and microtopography).

Adult facilitation becomes competition as juvenile soapberry bugs age.

Intraspecific interactions can change from facilitative to competitive depending on the organism's ontogeny. In plant‐feeding insects, host plant defenses can be strengthened or weakened by insect feeding and can therefore be important for determining whether two insects feeding on the same plant help or harm each other's fitness. Here, I conducted two experiments looking at the direct effect of a physical seed defense and the role of intraspecific facilitation in reducing the effects of that defense for juveniles of the red‐shouldered soapberry bug. I demonstrate that juveniles are severely inhibited by the tough seed coat of their host plant, leading to high mortality early in development. Adults, in contrast, can create holes through which younger individuals could potentially feed. I manipulated whether or not seeds were fed on by adults on two host plant species: a well‐defended native host and a poorly defended introduced host. Survival in the first week of development was dramatically improved by prior adult feeding, and this facilitation was stronger on the well‐defended host plant. However, the benefits of prior adult feeding ceased after the first week of development and shifted to having a negative effect on survival, development time, and body size. These results indicate that ontogeny is a key factor determining the effects of plant defenses and the strength and direction of intraspecific interactions across multiple host plant species.

Spatial Analysis as a Tool for Plant Population Conservation: A Case Study of Tamarix chinensis in the Yellow River Delta, China

Saltcedar (Tamarix chinensis) is undergoing population declination and fragmentation due to climate change and human disturbance. The existing restoration strategies usually focus on improving the environmental conditions based on the environment–saltcedar relationship, while they ignore the role of spatial autocorrelation resulting from biological interaction and ecological processes. This oversight limits the efficiency and sustainability of the restoration. Here, we explored the spatial pattern of the saltcedar population in the Yellow River Delta, China, and its relationship with environmental factors, incorporating spatial autocorrelation. The plant and soil parameters were extracted by an airborne LiDAR system integrated with fixed soil environment measurements. The environment–saltcedar relationship incorporating spatial autocorrelation was evaluated with different regression models. Results showed that saltcedars aggregated at small scales (2–6 m), resulting from intraspecific facilitation and wind dispersal of seeds, while intraspecific competition was responsible for the random distribution at large scales (>10 m). The long-distance dispersal of seeds through water explained the significant positive spatial autocorrelation of saltcedars at distances up to 125 m. Consequently, resulting from intraspecific facilitation and seed dispersal, aggregation distribution and positive spatial autocorrelation within the saltcedar population improved the adaptability of saltcedar to environmental stress and thereby reduced the impact of environmental factors on the abundance of saltcedar.

Facilitation in plants shifts along a recovery gradient induced by long‐term overgrazing in a temperate steppe community

AbstractQuestionsPlant facilitation often dominates over competition in stressful environments. However, to our knowledge few studies have examined plant facilitation within the context of herbivore pressure. Herbivory‐induced is a classic example of indirect facilitation. For example, in the temperate steppe region of Inner Mongolia, China, the intensive grazing by domestic animals has induced widespread grassland degradation. Here, we ask whether such long‐term overgrazing leads to facilitation in plants.LocationXilingol steppe region, Inner Mongolia, China.MethodsWe examined the spatial structure of three dominant plant species, including two species that are palatable to sheep (Leymus chinensis and Agropyron michnoi) and one that is unpalatable (Stipa grandis). We examined whether the spatial aggregation of these three species provided evidence for facilitation by comparing their spatial patterns along a recovery gradient: high grazing pressure (HG), an eight‐years grazing exclusion (early‐stage recovery from grazing; ER), and a 21‐years grazing exclusion (late‐stage recovery; LR). To accomplish this, we used the bivariate pair‐correlation function, g12(r), to assess the spatial association between unpalatable and palatable species. A positive spatial association indicates that facilitation dominates over competition (or dispersal limitation, habitat heterogeneity). We used a null model to assess the significance of spatial associations. Furthermore, we evaluated intraspecific spatial aggregation with the g11(r) by combining to three null models at fine spatial scales.ResultsThe spatial association between palatable and unpalatable species suggests a hump‐shaped relationship along the gradient of the grazing exclusion time. Specifically, Leymus chinensis and Stipa grandis showed a neutral, positive and negative association across the three stages of natural recovery from 21 years of grazing exclusion to current grazing. Similarly, Agropyron michnoi and Stipa grandis showed a negative, very positive and positive spatial association along the same gradient. Moreover, intraspecific spatial aggregation was higher in the current grazing treatment than in the grazing exclusion treatment. This was particularly true at the finest spatial scales (i.e., <0.2 m).ConclusionsOur results suggest that facilitation between and within species along the recovery gradient in steppe communities is related to herbivore pressure induced by long‐term overgrazing. First, we observed no evidence of indirect facilitation between species in current grazing treatment, while direct facilitation between plant species appears strongest at early stages of natural recovery along the gradient of the grazing exclusion time. Moreover, we observed indirect intraspecific facilitation dominated in the current grazing treatment, especially at small scales across the same gradient.

Root exudate signals in plant-plant interactions.

Plant-to-plant signalling is a key mediator of interactions among plant species. Plants can perceive and respond to chemical cues emitted from their neighbours, altering survival and performance, impacting plant coexistence and community assembly. An increasing number of studies indicate root exudates as key players in plant-to-plant signalling. Root exudates mediate root detection and behaviour, kin recognition, flowering and production, driving inter- and intra-specific facilitation in cropping systems and mixed-species plantations. Altered interactions may be attributed to the signalling components within root exudates. Root ethylene, strigolactones, jasmonic acid, (-)-loliolide and allantoin are signalling chemicals that convey information on local conditions in plant-plant interactions. These root-secreted signalling chemicals appear ubiquitous in plants and trigger a series of belowground responses inter- and intra-specifically, involving molecular events in biosynthesis, secretion and action. The secretion of root signals, mainly mediated by ATP-binding cassette transporters, is critical. Root-secreted signalling chemicals and their molecular mechanisms are rapidly revealing a multitude of fascinating plant-plant interactions. However, many root signals, particularly species-specific signals and their underlying mechanisms, remain to be uncovered due to methodological limitations and root-soil interactions. A thorough understanding of root-secreted chemical signals and their mechanisms will offer many ecological implications and potential applications for sustainable agriculture.

Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem.

Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.

Optimizing coastal restoration with the stress gradient hypothesis.

Restoration efforts have been escalating worldwide in response to widespread habitat degradation. However, coastal restoration attempts notoriously vary in their ability to establish resilient, high-functioning ecosystems. Conventional restoration attempts disperse transplants in competition-minimizing arrays, yet recent studies suggest that clumping transplants to maximize facilitative interactions may improve restoration success. Here, we modify the stress gradient hypothesis to generate predictions about where each restoration design will perform best across environmental stress gradients. We then test this conceptual model with field experiments manipulating transplant density and configuration across dune elevations and latitudes. In hurricane-damaged Georgia (USA) dunes, grass transplanted in competition-minimizing (low-density, dispersed) arrays exhibited the highest growth, resilience to disturbance and dune formation in low-stress conditions. In contrast, transplants survived best in facilitation-maximizing (high-density, clumped) arrays in high-stress conditions, but these benefits did not translate to higher transplant growth or resilience. In a parallel experiment in Massachusetts where dune grasses experience frequent saltwater inundation, fewer transplants survived, suggesting that there are thresholds above which intraspecific facilitation cannot overcome local stressors. These results suggest that ecological theory can be used to guide restoration strategies based on local stress regimes, maximizing potential restoration success and return-on-investment of future efforts.

Putting self‐organization to the test: labyrinthine patterns as optimal solution for persistence

Spatial patterns formed through the process of self‐organization are found in nature across a variety of ecosystems. Pattern formation may reduce the costs of competition while maximizing the benefits of group living, and thus promote ecosystem persistence. This leads to the prediction that self‐organizing to obtain locally intermediate densities will be the optimal solution to balance costs and benefits. However, despite much evidence documenting pattern formation in natural ecosystems, there is limited empirical evidence of how these patterns both influence and are influenced by tradeoffs between costs and benefits. Using mussels as a model system, we coupled field observations in mussel‐culture plots with manipulative laboratory experiments to address the following hypotheses: 1) labyrinthine spatial patterns, characteristically found at intermediate to high patch densities, are the most persistent over time; this is because labyrinthine patterns 2) result in adequately heavy patches that can maximize resistance to dislodgement while 3) increasing water turbulence with spacing, which will maximize food delivery processes. In the field, we observed that labyrinthine ‘stripes’ patterns are indeed the most persistent over time, confirming our first hypothesis. Furthermore, with laboratory experiments, we found the ‘stripes’ pattern to be highly resistant to dislodgement, confirming the second hypothesis. Finally, with regards to the third hypothesis, we found positive effects of this pattern on local turbulence. These results suggest that the mechanisms of intraspecific facilitation not only depend on initial organism densities, but may also be influenced by spatial patterning. We hence recommend taking into account spatial patterns to maximize productivity and persistence in shellfish‐cultivation practices and to increase the restoration success of ecosystems with self‐organizing properties.

Intraspecific facilitation explains the persistence of Phragmites australis in modified coastal wetlands

AbstractCoastal exploitation and human‐mediated modifications have markedly altered the community composition and functioning of coastal wetlands worldwide. Although recent work has shown that harnessing positive density‐dependent feedbacks can greatly enhance the recovery of habitat‐modifying species in degraded wetlands, the role of these intraspecific feedbacks in explaining the persistence of altered, unfavorable plant communities remains largely unexplored. Here, we experimentally tested whether intraspecific facilitation may explain the persistence of common reed (Phragmites australis) in human‐modified coastal wetlands. We performed a full‐factorial mesocosm experiment crossing low‐density pioneer versus high‐density established development stages with saline (20 psu) versus freshwater conditions. Results showed a clear shift in plant growth response from intraspecific competition under freshwater conditions to self‐facilitation in saline treatments. We identified two positive feedback mechanisms enabling the established treatment to overcome salinity stress: (1) Enhanced root oxygenation of the sediment at higher plant density decreased accumulation and intrusion of phytotoxic sulfide, and (2) density‐dependent rainwater infiltration into the soil lowered salinity in the dense root mat, preventing salt stress. Our study demonstrates that intraspecific facilitation can be an important factor in explaining the persistence of Phragmites australis in coastal wetlands. We emphasize the importance of integrating positive interactions in coastal restoration but argue that they should either be harnessed when restoring vegetation or broken when eradication of nontarget species is the management objective.

Intraspecific facilitation explains the spread of the invasive engineerSpartina anglicain Atlantic salt marshes

AbstractQuestionsInvasiveness depends in part on the ability of exotic species to either exclude native dominants or to fill an empty niche. Comparisons of niches and effects of closely related native and invasive species enable the investigation of this topic. DoesSpartina anglicainvade European salt marshes through competitive exclusion of the nativeSpartina maritimaor due to the occurrence of an empty ecological niche in highly anoxic conditions?Study SiteThe Arcachon Bay (France).MethodsAt three intertidal levels, we quantified competitive response and effect abilities of the two species through a cross‐transplantation removal experiment. We also compared the biomass, root/shoot ratio, productivity and environmental conditions (elevation, salinity, redox potential and soil moisture) of salt marsh communities dominated by the exoticSpartina anglicaor the nativeSpartina maritimaat three intertidal levels.ResultsBoth established species showed similar biotic resistance to the invasion of the other species, but the exotic showed important intraspecific facilitation for growth. Species had similar niches and total biomass along a gradient of anoxic conditions, but the exotic had a much higher root/shoot ratio and productivity than the native. Owing to its rhizome density, the exotic showed high ability to increase sediment oxygenation, likely to explain its important intraspecific facilitation.ConclusionsOur results showed that the invasion success ofS. anglicacannot be explained by the competitive exclusion of the native or by its ability to fill an empty niche along a gradient of anoxia. Its behaviour as a self‐facilitator invasive engineer very likely explains its rapid spread in the Bay and biotic resistance to the colonization of other congeneric species when established in dense patches. Additionally, we suggest that physical disturbance in marsh communities dominated by the nativeS. maritimamay disrupt its biotic resistance against the invasion ofS. anglica.

Conspecific plants are better 'nurses' than rocks: consistent results revealing intraspecific facilitation as a process that promotes establishment in a hyper-arid environment.

Harsh environmental conditions in arid ecosystems limit seedling recruitment to microhabitats under nurse structures, such as shrubs or rocks. These structures, however, do not necessarily afford the same benefits to plants because nurse rocks provide only physical nurse effects, whereas nurse plants can provide both physical and biological nurse effects. Nevertheless, if the nurse plant is a conspecific, the benefits it provides may be outweighed by higher mortality due to negative density-dependent processes; consequently, negative density-dependence is expected to limit plants from acting as nurses to their own seedlings. The degree to which an abiotic nurse may be more beneficial than a conspecific one remains largely unexplored. Here, we examine the role and elucidate the mechanisms by which conspecific plants and rocks promote plant establishment in a hyper-arid desert. For 4 years, we examined establishment patterns of Myrcianthes coquimbensis (Myrtaceae), a threatened desert shrub that recruits solely in rock cavities and under conspecific shrubs. Specifically, we characterized these microhabitats, as well as open interspaces for comparison, and conducted germination, seed removal and seedling survival experiments. Our results revealed that conspecific shrubs and nurse rocks modified environmental conditions in similar ways; soil and air temperatures were lower, and water availability was higher than in open interspaces. We found no evidence on negative density-dependent recruitment: seed removal was lowest and seedling emergence highest under conspecific plants, moreover seedling survival probabilities were similar in rock cavities and under conspecific plants. We conclude that the probability of establishment was highest under conspecific plants than in other microhabitats, contrasting what is expected under the Janzen-Connell recruitment model. We suggest that for species living in stressful environments, population regulation may be a function of positive density-dependence and intraspecific facilitation may be a process that promotes the persistence of some plant species within a community.

Spatial patterns of ponderosa pine regeneration in high-severity burn patches

Abstract Contemporary wildfires in southwestern US ponderosa pine forests can leave uncharacteristically large patches of tree mortality, raising concerns about the lack of seed-producing trees, which can prevent or significantly delay ponderosa pine regeneration. We established 4-ha plots in high-severity burn patches in two Arizona wildfires, the 2000 Pumpkin and 2002 Rodeo-Chediski Fires, to determine if: (1) distance from forest edge influences the density and spatial patterns of regenerating ponderosa pine and sprouting tree species, (2) interactions with re-sprouting trees affect spatial patterns of ponderosa pine regeneration, and (3) distance from forest edge and species competition affect regenerating ponderosa pine height. Plots were located in high-severity burn patches (defined as 100% tree mortality) and either adjacent to residual live forest edges (edge plots), or >200 m from any residual live trees (interior plots). We found higher ponderosa pine regeneration densities in the edge plots (13–154 (median = 69) stems ha−1) than the interior plots (12–124 (median = 29) stems ha−1) on both wildfires, but no differences in spatial patterns between edge and interior plots. Ponderosa pine regeneration displayed patterns of small-scale spatial aggregation in all plots, except one edge and one interior plot on the Pumpkin Fire, which displayed random distributions. These patterns suggest both short- and long-distance dispersal play important roles in ponderosa pine regeneration in high-severity burn patches. Sprouting trees dominated tree regeneration on the Rodeo-Chediski Fire, but they were spatially independent of ponderosa pine and did not influence ponderosa pine height. Regenerating ponderosa pine height was positively correlated with neighboring ponderosa pine densities and height, suggesting that intraspecific facilitation or similar habitat preferences occur in high-severity burn patches. Collectively, these results indicate that ponderosa pines are re-establishing with heterogeneous spatial patterns in large high-severity burn patches, but often with low densities. Also, ponderosa pine regeneration could be more strongly influenced by intraspecific facilitation than interspecific competition from dense sprouting species. Future forest spatial patterns and composition are still unclear, but at this stage of development, these heterogeneous patches, characterized by drought-tolerant sprouting species or low pine densities, could be more resilient to climate change and severe wildfires than the overly-dense ponderosa pine forests that were present before the wildfires.

Density-dependent shift from facilitation to competition in a dwarf Avicennia germinans forest

The global effort to rehabilitate and restore destroyed mangrove forests is unable to keep up with the high mangrove deforestation rates, which exceed the average pace of global deforestation. Although facilitation theory presents new possibilities for the restoration of heavily degraded mangrove sites, knowledge of tree–tree interactions in stressed mangrove forest ecosystems is too limited to utilize facilitation appropriately. The aim was to determine the mode of local interaction among stressed mangrove trees by investigating the effect of clustering on tree size and crown morphology under contrasting stand densities. The study was conducted in a dwarf Avicennia germinans forest in Northern Brazil, in which tree growth is limited by infrequent inundation and high pore-water salinity. Autoregressive regression, Voronoi tessellation and spatial point pattern statistics were used to address the spatial processes underlying tree interaction. Under low stand density (1.2 trees m−2) dwarf trees which grew in clustered cohorts of A. germinans had a less stunted crown morphology revealing the dominance of a positive neighborhood influence among plants. In contrast, dwarf trees in the denser forest stand (2.7 trees m−2) were interacting competitively as indicated by the more negative effect of neighbors on crown morphology and size. The shift from facilitative to competitive interactions is an important feature of mangrove forest regeneration under harsh environmental conditions. If mangrove trees are unable to regenerate naturally on severely degraded sites, intraspecific facilitation could be used to assist regeneration by planting seedlings in clusters and not evenly spaced.

Field manipulations of resources mediate the transition from intraspecific competition to facilitation.

Population density affects individual performance, though its effects are often mixed. For sessile species, increases in population density typically reduce performance. Still, cases of positive density-dependence do occur in sessile systems and demand explanation. The stress gradient hypothesis (SGH) predicts that under stressful conditions, positive effects of facilitation may outweigh the negative effects of competition. While some elements of the SGH are well studied, its potential to explain intraspecific facilitation has received little attention. Further, there have been questions regarding whether the SGH holds if the stressor is a resource. Most studies of interactions between the environment and intraspecific facilitation have relied on natural environmental gradients; manipulative studies are much rarer. To test the effects of intraspecific density and resources, we manipulated resource availability over natural population densities for the marine bryozoan Watersipora subtorquata. We found negative effects of density on colony performance in low resource environments, but mainly positive density-dependence in high resource environments. By adding resources, competition effects were reduced and the positive effects of facilitation were revealed. Our results suggest that resource availability mediates the relative strength of competition and facilitation in our system. We also suggest that intraspecific facilitation is more common than may be appreciated and that environmental variation may mediate the balance between negative and positive density-dependence.